The effect of high heat and drought on corn

High temperatures can cause problems in corn even when soil moisture is adequate — and will compound problems in drought-stressed corn. Hot, dry conditions are particularly damaging during pollination (VT-tassel through R1-silk). Much of the corn crop in Kansas is just now entering this critical period for determining grain yield.

Effects of stress at pollination time

There are several reasons why the four weeks centered around pollination are so critical for determining grain yield. During the last couple of weeks before tassels emerge, the potential ear length is being determined. Extreme stress at this time can reduce the number of kernels per row – affecting potential ear size. Extremely high temperatures prior to and during pollen shed can reduce pollen viability.

Drought stress can slow silk elongation so much that the pollen may be shed before the silks emerge. Lack of water can also result in poor tassel exertion. Combined with the leaf rolling associated with drought stress, the pollen may be shed before the tassel has emerged. Even if pollination does occur successfully, kernels may abort during the first several days of development under severe heat/drought stress. All of these factors can reduce successful pollination, kernel set, and kernel development, reducing the number of kernels per acre – the greatest determinant of grain yield.

Management options for stressed corn

Where dryland corn has been under severe drought stress, you’ll have to decide whether to let it go and hope for some kind of grain yield, salvage the crop for silage or hay, or leave the crop in the field for its residue value. It likely will pay to wait until after pollination is complete before making this decision to get some idea of kernel set. If kernel set is good, the ears at least have the potential to produce grain. If kernel set is severely reduced, the first step is to estimate potential grain yield based on kernel numbers per acre and average to slightly below average kernel size. This can help you make the grain vs. forage decision.

Economically, should you leave the corn or cut it for silage or hay, or leave it for residue? The value of the residue for moisture retention, soil quality, and future crop productivity will vary depending on the situation, and can be hard to quantify -- but it is considerable. As for the silage/hay vs. grain decision, if the yield potential is less than 25 bushels per acre, it’s probably best to cut it for silage or hay. If the yield potential is 50 bushels or more, it’s probably best to harvest it for grain. If the yield potential is between 25 and 50, the decision will depend on the price of corn, the quality of the silage, and on a producer’s ability to use or sell the silage.

Of the two options for dryland corn that has limited yield potential – silage or hay -- silage is normally the preferred option. However, you need the facilities to make silage (or sell it to someone who does), and there must be enough moisture in the plants to properly ensile. And where there’s no ear at all, silage may not be a good option. Where the ear is very small, or has poor seed set, the silage will have lower energy value (TDN) and lower overall forage quality than normal. Even at normal yield levels, silage quality begins to decline when grain yield drops below roughly 150 bushels per acre, and continues to decrease as grain yields keep going down.

To cut corn for silage, you need 65 to 75 percent moisture in the plant. If plants are suffering from drought, they may have lost some of the bottom leaves. The top leaves may have browned off or turned white. In that case, the plants probably do not have 65 percent moisture, depending on how much moisture is in the stalk.

Where that’s the case, your only option is probably to chop and graze, or hay the crop like a summer annual forage. The pasture/hay shortage that exists in some areas of the state may make haying the failing corn crop a more desirable option this year.

When chopping or cutting for hay, stalks should be cut at least six to eight inches off the ground to avoid nitrate toxicity that may result when feeding forage made from drought-stressed corn. Under drought conditions, the plant does not grow normally and high levels of nitrate can accumulate, especially in the lower portions of the stalk. You should also have corn hay (or stubble if you plan to graze) tested for nitrates. A forage nitrate test costs only $5-15 and it’s the only sure way to make sure the hay is okay to feed to cattle. Ensiling the corn, if possible, is preferred to chopping or grazing because of that potential for nitrate toxicity.

If you plan to have cattle graze the corn field after it has been chopped or cut for hay or silage, watch for any shattercane or Johnsongrass that comes up after a rain. New regrowth from these sorghum-type plants after a drought can be dangerously high in prussic acid.

How much silage can producers get from drought-stressed corn? A publication from the University of Wisconsin estimates that for corn that has been stressed, with limited grain yield potential, producers can expect about one ton of silage per acre for every five bushels of grain yield. For corn that is not stressed, producers can get about one ton of silage for each six to seven bushels of grain yield. If little or no grain is expected, a very, very rough pre-harvest estimate of yield can be made by assuming that one ton of silage can be obtained for each foot of plant height, excluding the tassel.

Putting a value on silage or hay

One of the biggest questions is how to put a value on the silage or hay. The price of corn silage is typically based on corn or hay prices. The K-State Farm Management Guide crop budgets value silage in the field at 8 times the price of corn, meaning that if corn is $6/bushel, the price of corn silage would be $48/ton ($6 x 8). However, drought-stressed corn typically will have a value of 75-95 percent of “normal” corn silage, so a more appropriate rule of thumb for drought-stressed corn silage standing in the field would be 6 times the price of corn.

Current projected costs for silage chopping, hauling, and silo filling range from $8.00 to $8.50/ton. Harvesting corn for grain is expected to cost $0.50 -$0.55 per bushel. Before any action is taken in regard to harvesting corn for grain, silage, or hay, producers should contact their crop insurance agent.